Single-cell levitation

A new technique, micro-DEP uses the fact that growing cells (e.g., fetal, tumor, would-healing) oscillate electrically. The cells emit non-uniform electric fields and can attract small polar particles by DEP. The technique consists of measuring the cells ability to collect tiny l-pm- 

FIGURE 9. Continuous DEP flow chamber and electrode system. 

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A clever way to minimize the unwanted background from the cover plate or the environment is combining laser tweezers and confocal Raman spectroscopy (LTRS) [73, 74], While the single cells are levitated well off the surface and held in the focus of the laser beam the Raman spectral patterns of these cells are recorded with high sensitivity. Another appealing fact is the usage of one laser for both Raman excitation and optical trapping to keep the instrumental efforts as low as possible. Additionally the trapped cells can also be micro-manipulated and moved from one place to another, e.g., from the native matrix to a clean collection chamber. 

Kaler, K. V. I. S. and Jones, T. B., Dielectrophoretic spectra of single cells determined by feedback-controlled levitation, Biophys J, 57, 173,1990. 

Hakoda, M., Hachisu, T., Wakizaka, Y., Mii, S., and Kitajima, N., Development of amethod to analyze single cell activity by using dielectrophoretic levitation, Biotechnol Prog, 21,1748, 2005. 

Wood et al. have combined acoustic levitation and Raman spectroscopy with the intention of developing a field tool for environmental monitoring of algal blooms and nutrient availability [62]. Heraud discussed the most appropriate spectral preprocessing techniques for analysis of Raman spectra of single microalgal cells and developed a method to predict the nutrient status of those cells from in vivo spectra [63,64]. 

Recently, Lepoint-Mullie et al. used Weissler s reagent and starch to trap iodine (levitation cells at 24 kHz or 43 kHz).164 They observed that single bubbles showed a strong chemical activity under certain dynamic regimes (i.e., not only when they luminesce) and the spatial distribution of the chemical species was strongly anisotropic (Fig. 33). Under the most chemically active conditions (with no SL detected by a photo-multiplier tube with sensitivity 3 x 10"H Im), over 10 chlorine radicals were released at each acoustic cycle. Such an experiment (with chemical species as tracers) also enables a study of the liquid flows around a bubble.165... 

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